Knee surgery for the repair and/or replacement of knee joints in a human subject has become a common place procedure. Total knee implants when properly installed will closely approximate or mimic natural knee movement. Heretofore, with the surgical instruments available, a surgeon installing an implant must have had a high degree of skill in order to achieve optimim fit and alignment.
The knee joint is formed between the condyles of the femur in the upper leg and the head of the tibia in the lower leg with the patella in front. The knee joint permits flexion and extension and in certain positions, a slight rotation inward and outward. This movement is not a simple hinge like motion but is a complicated movement consisting of a certain amount of gliding and rotation.
In order for the knee to function with the required range of movements, the alignment of femur to the tibia is of key importance. Incorrect alignment results in inappropriate contact between the femoral condyles and the tibia causing abnormal wear on contact points of the bone with resulting malfunction of the joint.
The alignment of the knee with respect to the hip and the ankle is determined by measuring the angle between the mechanical axis of the whole leg and the longitudinal axis of the femoral shaft, the angle being about 6.degree.. The mechanical axis is defined as a straight line extending from the center of the hip, through the center of the knee to the center of the ankle along the tibial shaft forming an angle of approximately 3.degree. to the vertical. In contrast, the femoral shaft meets meets the center of the knee at an angle of about 9.degree. to the vertical, this angle varying according to body build. When the natural alignment of the knee joint is disrupted, subsequent joint instability, ligamental imbalance and excessive stress at fixation interfaces occurs. Correction of this type of damage necessitates in some cases a total surgical replacement of the knee joint. Just as the initial damage to the joint is caused by misalignment, the successful replacement of the knee joint calls for the optimum alignment of the prosthesis. At present, these alignment measurements commonly rely on indirect measurement of the angle between the longitudinal axis of the femoral shaft and the mechanical axis by measuring each angle relative to the vertical or horizontal axis (U.S. Pat. No. 4,349,018; U.S. Pat. No. 4,487,203).
The surgical procedure for knee joint replacement involves a series of steps that include opening up the knee joint, followed by the cutting and shaping of the proximal tibia and the distal femur and the subsequent insertion of commercially available knee prosthesis. The successful positioning of the prosthesis is dependent on all the preceding surgical steps. These steps not only include the proper alignment of the knee during the cutting and shaping of the bone prior to the application of the prosthesis but also on the proper alignment during placement and fixation of the prosthesis trials.
Because existing alignment procedures rely on alignment guides that form part of the drill jigs used to cut and shape bone (U.S. Pat. No. 4,759, U.S. Pat. No. 4,349,018; U.S. Pat. No. 4,487,203 and U.S. Pat. No. 4,567,885), no direct alignment measurement after removal of the jigs is possible. Thus, alignment of the femur and tibia may be determined during cutting the bone but not during subsequent fitment of the prosthetic device.
Guide rods of the prior art are fixed in the tibia or femur by drilling a plurality of holes into the bone of the tibia or femur and utilizing pins inserted into the holes to secure the guide. The guide rods of the prior art are secured throughout the procedure for cutting and shaping the femoral condyle and the tibial head. (U.S. Pat. No. 4,487,203; U.S. Pat. No. 4,567,885; U.S. Pat. No. 4,759,350; U.S. Pat. No. 4,349,018, U.S. Pat. No. 4,621,630). Indeed, the use of prior art alignment guides is restricted to the measurement of alignment during cutting and shaping of the femoral condyle and the tibial head. (U.S. Pat. No. 4,944,760; U.S. Pat. No. 4,621,630; U.S. Pat. No. 4,759,350) and cannot be used to check alignment of the knee during placement of the tibial and femoral trials. In some cases, separate and distinct tibial and femoral alignment guides are used, requiring holes to be drilled in the medullary canal of both the femur and the tibia of a subject. (U.S. Pat. No. 4,349,018; U.S. Pat. No. 4,567,885).